German Offenlegungsschrift No. 26 45 788 discloses a mechanized welding system having a welding torch which is periodically swung laterally (transversely) of the welding seam. The welding current is measured at the laterally outermost points (turning points) of the swinging movement and, based on a comparison of the two values obtained, the welding head is, if necessary, moved so as to be more accurately centered over the seam. A similar method, namely measuring the welding-current strength and then comparing this value with a predetermined value, is utilized for adjusting the welding torch elevationally of (toward or away from) the seam. It is possible for the latter comparison to use the welding current at the seam center. The advantage of this lateral and elevational scanning is that the welding torch can automatically be guided at a predetermined elevation above and laterally along a welding seam which is not known in its course. However, it is disadvantageous that this manner of guiding the welding torch requires a constant swinging of the same, which causes the welding speed along the welding seam to be considerably slowed down.
Furthermore, mechanized welding systems are known in which the welding torch is moved according to a predetermined program which corresponds with the anticipated course of the seam. The advantage of this type of control of the welding torch is that swinging movements to determine the seam center are not needed, which results in a high welding speed. However, it has been found in practice that, for satisfactory functioning of such a welding system, the actual course of the seam must correspond exactly with the preprogrammed one. If deviations occur, as can easily happen in practice due to alignment errors or due to tolerances, then there exists the risk that the preprogrammed welding seam is positioned next to the actual seam or only partially covers it, so that the resulting weld is insufficient and perhaps faulty. A particular problem involves locating the start of the seam. Based on the above-mentioned alignment and tolerance errors, it is often the case that the start of the seam remains unwelded.
A basic purpose of the invention is therefore to provide a method for locating and welding preprogrammed welding seams, with which the beginning of the seam can be located and welded satisfactorily and in which a satisfactory weld can be achieved even in the case of positioning errors and/or deviations due to tolerances in the seams to be welded.